PROJECT ABSTRACT Human inflammatory bowel disease (IBD) is characterized by inconsistent response to therapies and persistent activation of pathogenic effector CD4+ T cells implying regulatory T cell (Treg) dysfunction; however, the underlying mechanisms are poorly understood. Therefore, the OVERALL OBJECTIVE of this proposal is to elucidate the mechanisms by which gastrointestinal cues impact Treg metabolism and function with the therapeutic goal of defining pharmacological and adoptive Treg therapies to treat IBD. For the first time using complementary approaches, we have observed a defined mitochondrial ultrastructure (shape, cristae structure, and physical interaction with the endoplasmic reticulum [ER]) which correlated with Treg metabolic state. We are now poised to exploit how: i) mitochondrial ultrastructure and its associated metabolic state in Tregs suppress gut inflammation in various mouse models of experimental colitis; and ii) a breakdown in the regulation and function of mitochondrial ultrastructure can drive human IBD pathogenesis by analyzing Tregs from peripheral blood and lamina propria of IBD patients compared to relevant healthy individuals. Our preliminary data suggest that anti-inflammatory transforming growth factor beta 1 (TGF-?1) cytokine is a critical driver of mitochondria-ER contact (MERC) in Tregs via its associated molecular architecture, thus implicating intact MERC and subsequent pyruvate oxidation in Treg-mediated suppression of IBD. In our preliminary experimentations mimicking the proinflammatory milieu of IBD gastrointestinal tract, treatment of Tregs with proinflammatory cytokines impaired MERC and perturbed glucose metabolism, leading to excessive fatty acid oxidation as a compensatory mechanism in contrast to vehicle-treated Tregs (?proinflammatory cytokine- induced metabolic reprogramming?). Furthermore, we discovered that proinflammatory cytokine-induced metabolic reprogramming of Tregs was reversed by inhibiting the activity of glycogen synthase 3 beta (GSK3?) using a class of inhibitors currently being explored in clinical trials for other indications. Based on these novel observations, we formulated the CENTRAL HYPOTHESIS that TGF-?1 mediates mitochondria-ER contact that is essential for cellular metabolic homeostasis, Treg function, and suppression of IBD pathogenesis. The following independent SPECIFIC AIMS are designed to test three integrated hypotheses. First, we will directly test the hypothesis that TGF-?1 mediates MERC and consequently Treg function. Second, we will test the hypothesis that TGF-?1 potentiates mitochondrial pyruvate oxidation and consequently Treg function. Third, we will test the hypothesis that proinflammatory cytokines perpetuate IBD pathogenesis in vivo via MERC inhibition. We propose to utilize sophisticated approaches relevant to health and IBD pathophysiology to test this hypothesis. This proposal, which is technically and conceptually innovative, is also significant because it presents a novel concept in Treg biology and identifies new mechanisms for therapeutically optimizing Tregs, namely combining pharmacological and Treg-based therapies, to halt the refractory nature of IBD.